Weight Painting Mirroring Issue
That mirror option is for a center bone that should control left and right side of a mesh at the same way (like a hip bone, a torso, and so on ...).

To have R side bone matching the L side (like arms or legs) this is one procedure:

Weight paint the L side (i.e. arm.L, the name of the vertex group must have an appropriate suffix, like .L or .l or .left).

Then select the option copy vertex group, then mirror vertex group, then rename appropriately (like arm.R).